Final answer:
The cash crop in Afghanistan that thrives on marginal soils and has a high economic return is c. opium, derived from poppies. It accounts for a significant portion of the Afghan income and is mostly exported to European markets. The country's economy is also poised to shift towards mining due to large deposits of valuable minerals.
Step-by-step explanation:
A cash crop in Afghanistan that does well on marginal soils yielding a high return for resources expended is c. opium, which is derived from poppies. Afghanistan is the world's largest producer of opium, a key ingredient in the production of heroin.
This cash crop has become a significant part of the Afghan economy, contributing to a notable portion of the country's income. The expansion of poppy cultivation and the subsequent drug trade may account for as much as one-third of Afghanistan's income, with more than 80 percent of the heroin consumed in Europe originating from Afghanistan.
The trade in opium poppies has also expanded to South America. In particular, Colombian drug cartels have started to cultivate opium poppies, which thrive on the higher and more arid slopes of the Andes.
However, the country has begun a transition with the discovery of vast mineral resources that include iron, copper, gold, and rare industrial minerals such as lithium, which are in high demand globally. This discovery offers the potential for a significant shift in Afghanistan's economy from agriculture to mining.